Add support for asyncio

[NEONKID]

This PR contains the core code prepared for asynchronous processing in Python.

Asynchronous processing using the asyncio module is supported from SQLAlchemy version 1.4.
(https://docs.sqlalchemy.org/en/14/orm/extensions/asyncio.html)

So, in order to use these asynchronous modules in testcontainers, we make a PR request for the code that supports these modules.

In addition to PostgreSQL, other relational DB drivers can also support it, and a parameter has been added to the function to select Dialect in the existing code.
(Python uses several DB driver, so I don't think it's good to hardcode a specific driver #140)

[NEONKID]

If you would like to provide feedback on this PR, feel free to leave a comment.

[tillahoffmann]

Do you think it's feasible to achieve this using metaprogramming to avoid repeated code (one implementation for async one for sync)?

[yakimka]

Are we need async implementation for testing purposes?

[tillahoffmann]

I'm going to close this PR for now. The main reason is that the benefits of async only manifest in the async implementation of sqlalchemy. In particular, none of the container-related code uses await syntax such that the code will run exactly the same way as if it was entirely synchronous. Users who want to make use async database connections should instantiate their own async connection.

Feel free to disagree, and we can discuss reopening this PR.